1. Technical Field
This invention relates to pointer checking in a database. More specifically, the invention relates to selecting an optimal record access method prior to opening any files.
2. Description of the Prior Art
Periodically databases need to be reorganized. This is generally due to that fact that databases tend to grow in an uneven manner resulting in a disorganized system with a commensurate degradation in performance. Reorganization of a database includes changing aspects of the logical and/or physical arrangement of the database. Over a period of usage, database management systems (DBMS) will require reorganization in order to restore database performance levels and maximize database capacity. During most types of reorganization, the area being reorganized is typically offline and unavailable to users. The data reorganization activity may result in data remaining unavailable for a substantial interval (i.e. hours or more). Therefore, it is desirable to reduce the amount of time the database is unavailable by reducing the number of steps involved in a database reorganization operation, and more specifically to reduce the number of steps involved in reorganizing a hierarchical database.
A hierarchical database management system, such an IBM's IMS DL/I (IMS) manages data in a tree structure. Each data element is called a segment. The first data element is called the root segment of the structure. The root segment is the top of the tree and each subordinate segment is a child of the root or is a child of the child of the root segment or is a child further descended down from the root segment of the tree. One database is analogous to a forest containing many trees all of which share the same defined segment structure.
IMS also utilizes keyed access directly to any segment in the database. Keyed access is provided through an index file containing pointers directed to the segments in the database. The use of the index allows for the locating of a desired element through the index as opposed to a sequential searching through a collection of elements. Indexing data by using direct pointers to data elements is common in databases managed by DBMS. It is well known in the art that a segment which otherwise can be pointed to directly, can also be pointed to indirectly through an index. However, indexing data is usually the result of a user defining the data for which an index is used, either beforehand or dynamically.
One difficulty in managing hierarchical data is that logical relationships may exist between data elements in the same database or between specific data elements in different databases. As such, direct pointing between data segments in the same or different databases is managed through a relative byte address (RBA). Each segment has direct pointers consisting of RBA to which a segment points. When the logical relationship among multiple databases is defined, an application program can access the databases as one database. When a secondary database is defined with an index, an application program can access the indexed database faster than without the indexing. Accordingly, direct pointers are used to maintain the logical relationship among databases.
A feature of the IMS provides the ability to partition each database thereby increasing storage capacity and improving availability. Each partition in the database may be separately reorganized when needed. During the reorganization of a partition, the RBA of a segment may be relocated. However, the direct pointer associated with the segment residing in a different database not subject to reorganization is not updated. To resolve this problem, an indirect pointer and reorganization number is provided. The reorganization number is the count of the number of times the partition has been reorganized. The indirect pointer contains the current RBA of the pointed segment. The indirect pointer is an intermediate pointer from the direct pointer to the current segment RBA. The direct pointer is related to the indirect pointer by an Indirect List Key (ILK). When a segment is inserted to a partition, the ILK is formed from the RBA of that segment, the current partition identifier, and the current partition reorganization number. As a result, the partitioned segment relies on the use of an ILK for various purposes.
The indirect pointers are stored in a file known as an Indirect List Data Set (ILDS), which is a key sequenced data set. A data element in the ILDS is named as in Indirect List Element (ILE). The ILE consists of the ILK, partition identifier, the reorganization number, and the indirect pointer. There is one ILE for every segment involved in inter-record pointing. The ILEs are created or updated during a database reorganization and kept at the current values. The direct pointer is stored in an extended pointer set (EPS), which is part of the segment prefix. The EPS consists of the identifiers of the pointing segment, which are the partition identifiers, the reorganization number of the partition, the direct pointer, and the ILK. It should be noted, the EPS could have an older reorganization number than the current reorganization number since the EPS is not updated during reorganization.
After a reorganization, IMS compares the reorganization number in the EPS with the current reorganization number in the partition. If the two reorganization numbers are the same, the direct pointer contains the current value of the reorganization number and the IMS does not need to refer to the ILE. However, if the two reorganization numbers differ, the IMS refers to the ILE for the current reorganization number and the segment RBA, and updates the EPS to the current value.
The ILDS is a key sequenced data set of a virtual storage access method. There are three categories of methods for accessing a key sequenced data set: sequential, direct, and skip-sequential. Generally, satisfactory performance can be obtained by using the sequential access method to access a large quantity of records sequentially. Similarly, direct access generally provides satisfactory performance for accessing random keys, and skip sequential access generally provides satisfactory performance for accessing portions of records sequentially.
As shown herein, the IMS contains many types of pointers in order to support the relationships among data sets. In order to maintain database integrity, the pointers of the database need to be validated. A pointer checker performs the pointer checking. It executes separated from the IMS. The pointer checker reads all segments in source databases and target databases, and validates the consistency of each pointer value and an associated segment RBA. The pointer checker also refers to the ILDS to check for the direct pointer, the indirect pointer, and the segment RBA in the same manner as the IMS. The prior art pointer checker cannot select the optimum access method for the ILDS before opening the ILDS, as such there is no established method for estimating an optimum access method to the ILDS without first reading the ILDS. Therefore, the pointer checker defaults to a sequential access method to read the ILDS as it lacks a method of determining an optimal access method without first accessing the ILDS.
During the pointer check process, a target partition cannot be reorganized because the ILDS must be static. However, as the database changes, so do the number of reference ILEs. As a result, selection of an optimal access method can significantly decrease the time associated with ILDS access. Therefore, there is a need to identify an optimal access method before reading the ILDS to improve pointer checking performance.